WASHINGTON — The United States has stepped up drone strikes and other military operations inside Pakistan after concluding that Pakistan does not intend to crack down on Afghan insurgents along its border, U.S. officials said.

There have been more than 20 strikes by CIA-operated drones since Sept. 1, counterterrorism officials said, the highest monthly total in the nearly 10 years since the U.S. began carrying out such attacks.

The focus of the recent U.S. operations has been the Haqqani group, the violent Afghan insurgent organization that has long used North Waziristan, just inside Pakistan’s border, as a base of operations.

In addition to the drone attacks, the U.S. military command in Kabul confirmed that NATO helicopters crossed into North Waziristan last Friday to pursue Haqqani fighters who had attacked a U.S. outpost along the border. More than 50 insurgents were killed, NATO said.

The move to step up U.S. attacks came after senior Pakistani officials made clear in recent months that they are reluctant to move against the Haqqani network, despite high-level U.S. pressure to do so, because they fear the militants will respond with terror attacks inside Pakistan, a senior U.S. military officer said.

“”The Pakistanis have told us at the highest levels that if they went on the offensive against the Haqqani network, Haqqani would turn against them,”” said a senior U.S. officer in Afghanistan who was not authorized to speak publicly on the matter.

The situation points up the complex calculations in Washington, Kabul and Islamabad as the Afghan conflict reaches a critical phase. For years, parts of the Pakistani security services have nurtured ties to militants groups, including with the Haqqani network, but now are under pressure from the U.S. to turn against these organizations.

Unless Haqqani’s organization is substantially degraded over the next year, the U.S. will not be able to reach its goals for reducing the strength of the insurgency in Afghanistan, said another senior U.S. military officer, who also was not authorized to speak publicly on the issue.

The decision to step up targeting of the Haqqani organization was made after U.S. Army Gen. David Petraeus, the top commander of coalition forces in Afghanistan, and other senior U.S. officials decided that doing so was one of the only remaining options for substantially weakening the group, the two officials said.

Haqqani operatives and the U.S. troops have frequently clashed along the border in recent months, including in five major attacks against U.S. outposts in eastern Afghanistan.

Major General John Campbell, the senior U.S. commander for the area, said in an interview that Haqqani attacks against U.S. forces have been ineffective. “”They can mass every once in a while but they are just no match,”” he said.

But Campbell acknowledged that it will be impossible to defeat the Haqqani group as long as its fighters can move back and forth across the border and its leadership has a sanctuary in Pakistan.

“”We’re taking out a lot of the low-level guys, and I think we’re getting some of the mid-level guys,”” he said. “”But the senior guys don’t come into Afghanistan.””

Pakistani officials also have said that the devastating floods in many areas of the country have forced them to divert troops to relief operations that might otherwise have been used for military operations along the border.

U.S. attack helicopters have launched three attacks into Pakistan in recent days — one in which the aircrafts crossed briefly into Pakistan airspace and two in which they fired across the border at insurgents in Pakistan, the NATO military command in Kabul said.

U.S. military officials said their rules of engagement allow NATO aircraft to act in self-defense against insurgents who have launched attacks against NATO or Afghan forces from Pakistani territory. The U.S. has said in the past that Pakistan has agreed to those rules, though Pakistani officials on Monday denied that such an agreement exists.

Of the 22 drone strikes that the U.S. has reportedly carried out in Pakistan during September, all but three were in North Waziristan, according to Year of the Drone, a web site that tracks press reports about the attacks.

Drone strikes in Pakistan have escalated dramatically this year, with 76 so far compared to 53 in all of 2009, according to the web site.

Among the militants reported killed in the recent spike in drone strikes was an Afghan named Saifullah, the first cousin of Sirajuddin Haqqani, who heads the Haqqani network, which was created by his father, Afghan warlord Jalaluddin Haqqani.

Another factor in the latest round of strikes, according to a U.S. official not authorized to speak publicly about sensitive intelligence matters, is a concern that there is an increased risk of a terrorist plot underway against Europe or the U.S.

“”It shouldn’t surprise anyone that links between plots and those who are orchestrating them lead to decisive American action,”” the second U.S. official said. “”The terrorists who are involved are, as everyone should expect, going to be targets.””

The link between the recent drone strikes and intelligence about possible terror strikes in Europe was first reported by the Wall Street Journal.

But U.S. officials also acknowledged that some of recent U.S. attacks are related not to the recent intelligence about attacks in Europe but to the overall goal of inflicting as much damage as possible on Haqqani and his network.